Investment composition



Xi Zv34513l1 UNITED STATES 5x Is Reference PATENT OFFICE INVESTMENT COMPOSITION Robert Neiman, Louisville, Ky., assignor to Edmund A- Steinbock, Louisville, Ky.

No Drawing. Application July 16, 1941, Serial No. 402,699-

Claims.

This invention relates to investment compositions for use in making refractory molds into which metals or alloys may be cast into objects 'of a predetermined size and shape.

model is encased in another mix of investment,

thus forming the complete mold. An entrance or crucible chamber is formed so that the molten metal may enter the sprue hole or holes and thence form the casting itself. This mold is heated to such temperature as will melt and carbonize the wax, remove all moisture present, and thence generally to a temperature as will promote expansion of the mold and provide a mold hot enough to permit the entering molten metal to reach recesses of the cavity, formed by the removal of the wax, and

before the said molten metal solidifies. The mold is thence placed in a suitable casting machine, the metal is melted, and then poured or cast under pressure. After the metal solidifies and the mold cools sufflciently it is broken apart and the casting 3o recovered.

In order to carry out this procedure with the best results, it is necessaary that the investment composition form a plastic mass that'lsi'a-dlly" raises intii the desired shape. The investment should have the property of forming a very smooth and refractory surface in contact with the wax pattern so that the cast denture will be as smooth as possible and free from reaction products between the molten metal and refractory. 40

The investment should provide about minutes working time while in the plastic state and then should harden sufdciently for safejuse' ififillfminutesglto'il hr. The strength of the investment at the end of this latter period should be such that it may be safely used without the necessity of drying or dipping in molten waxes. Upon setting, the surface should not become unduly powdery and should promote adherence of the wax as much as possible.

For casting dentures, it is most desirable to have high thermal expansion, that is, between 1 and 2%, between room temperature and casting temperature. While being heated, the mold should expand fairly regularly and at no time should shrink appreciably below its original size. At the-same time, the mold should not crack durillg the heating period.

when making the original model upon which the. denture is built in wax), the investment should not change appreciably in size upon setting. Its setting expansion should be preferably between 0.0% and 0.1%. While slight shrinkage and positive expansions up to 0.2% are not extremely harmful, they, nevertheless, tend to produce undesirable distortion in the models.

The principal ingredient of an investment is the refractory flller, since it is this material that prouses the refractory properties necessary and promotes desirable expansion of the mold. For some purposes, it is desirable to have greater refractoriness without expansion. For other purposes, for example, in dentistry, it is necessary to provide high refractoriness with high thermal ex:- pansion.

One of the objects of this invention is to provide a composition of materials to meet all of the above mentioned exacting specifications.

Another object of this invention is to provide investments of practicallyzero setting expansion and with almost any desired thermal expansion.

Another object is to provide an improved binder for use with a wide variety of refractory fillers.

Still another object of this invention is to pro.- vlde a bond that will be very strong at room temperature and yet will not become much stronger nor vitrified on heating so that it may be removed from the casting without injuring the latter.

As was mentioned above, the principal ingredient of an investment composition is the refractory filler, and the one most generally used, is silica bining good expansion with low price and a wide variety of particle sizes.

Clays, escil u a rfr gggg, a wid used in industry but are not so satisfactory for the molds described herein when .45, used in large quantities due to their shrinkage or low expansion. They may be used where this disadvantage is not very material. Aluminum, oxide in its various mra'gegand, better yet, in tfie calcined and fused state makes a-good refraggigy. "Other refractories that may be used alone 0 lTU'Eore, (chieflylimfim and magnetite) other in a wide'variety of mixtures include refractory natural ores, olivin forst'erite," cordierite, pyroxenes, s inels, berillium oxide, cerium" oxide, lanthanum xide, thoric oxide, yttrium'r conium. 'I'Ee m' r'i'des an mentioned metals also are quite refractory. Car'- rugflt. art-i4 bon (especially as ra hite) and chromite, are 'ddfiiable refractories. em ers of the sillirnanite u .are valuable for their typical grain structure and expansion upon heating. Of these,

decomposes upon expansion to form mullite at approximately 1200 C. and above, and liiTte at 1300" 0., and sillimanite at about 15 O 5., all of these forming mullite and a fairly refractory glass. Mullite, itself does not expand appreciably on heating, but does not decompose until it reaches 1800 C. and thus is a very efileient refractory.

as described in co-pending application, Serial No. 309,784, is the highest expanding material known that possesses a high enough melting point for refractory properties to be useful in the casting of metals, such as alloys of chromium, nickel and cobalt and the like as now used to provide dentures. The use of 1 to of it is generally suiilcient t o p r c ivide adequate m s .eadret pra i iei hsie iififesi;s r aqi rxa e i qneqab va "6ther materials that may log used to promote expansion mins o ,"pyrophyllite, and baddelezit'efwhich undergo considerable expansion upon nversion. A number of other silicates undergo slight expansion on inversion. Many new refractories are continuously being made by mixing various proportions of oxides, especially those mentioned above, with each other, and firing them to various temperatures, thus producing refractories that combine the good properties of each.

The refractory aggregate should be properly ,ba ncedss to particlesiguo as to give strength to the investment mold as well as to resist rapid changes in temperature. Strength is achieved by properly grading t'ne refractories as to large, medium, and small particles, each of which fill the interstices between the next larger groups.

This maximum density gives strength, but too great a density is not desired, since this causes cracking and also lowers the permeability. Permeability is increased by using a more uniform grading of refractories. Also, the larger the mold" size, the larger the coarser material should be. For the usual dental molds, 35 mesh material is generally the coarsest used and with this, 200 mesh material or finer is used. For larger molds,

composition is extremely limited and in order to overcome this limited condition. a mold should permit casting of metals of higher melting point.

A number of binders are available which do not contain plaster and which generally comprise magnesia and an acid, such as we acid, as a 0 en practised. The use of sodium s date promotes vitrification and thus reduces refractoriness and produces a mold that is very diflicult to break up and separate from intricate I castings. The use of magnesium chloride, and Wis generally attended with dimensional- .c anges on setting that vary considerably and also this setting expansion may take place some time after the mold has been made. The use of magnesia and phosphoric acid is also subject to an undesirable wide limit of setting expansion. On one occasion it may be very low, approximately 2%, while the same mix under substantially the same conditions may expand up to 1.%. This is highly undesirable when extremely accurate work, such as the production of dentures, is to be done and also unusual setting expansions are attended with warpage and cracking of the material.

The use of alumin such as the hydroxide or Wwith Ehosphonc acid generally produces a large shr age on setting that is extremely detrimental. The use of metallic zirconates, such as mg;-

nesi 'rcona an a so the respective titanates as well as double zirconates, such as magt um, zinc, aluminum, sodium, otassium litfiiumalo'iigwiffi acid s such as iphosphoric and other mineral acids is also sub ee 0 a wide -variety of expansions and contractions on set- It has been found that a very refractory binde;\

and one giving the desired working time and without the undesirable features of unusual variation in the setting expansion, gas evolution,

obviously, very coarse refractories, in lump form or crushed rocks, would be desirable in order to preserve the mold shape when heated and prevent cracking and distortion.

From the large number of refractory materials, mentioned as well as similar ones or mixtures thereof (mechanical and chemical), a s ws. can. bed ..saasi ..eonbinagliasiisa alfdesirable features'for an investment to be utilized for thecasting of metals covering prac alloys containing nickel. When casting metals having a fusing point not much over 2000 F., there is a possibility of producing good castings in sul hate or plaster is used as used in amounts of 3% to 10% along w amoun 's' c this solution used to form a plastic mass with the refractory and magnesia mixture.

,' and sometimes up i To this must be added al%ifium in very small amounts, being .02% to to 1% of the total mixture. This should be preferably dissolved in the liquid, using either the -m tal, the hydroxide, h drated oxid or even the hospha e. ese are somewhat less suitable when added to the powder since some of them will not dissolve in time to promote the necessary a reaction previous to the final setting of the mix.

The amount of aluminum used depends upon the setting expansion desired. The mixture without aluminum when formed into a plastic mass M w .70 theplas tgr seems to be especially detrimental to and permitted to set will shrink slightly and then expand rapidly, generally in vary amounts within wide limits. If a eg ys r nall amount of aluminum, such as .05% be added, the mixture will gefie'fally shrink slightly and then expand just enough to make the total setting expansion approximately zero or neutral. Furth egadd itions of aluminum will cause the mEErial to shrink somewhat ar id plaster molds. It is, however, true that the alloy remain at this figure. Greater additions seem we. COMPOSlTlONS, I

COATING R PLASTIC are undesirable. The amount of aluminum needed depends upon the'type of magnesia used, its :grind,.and also upon the gradation and size Cross Reference proved by the use of most .reiractorymaterials, such as alumina, silicon carbides, zircon, and the siilimanit rou The Selfig time may be controlled 'by properly of refractory as well as .on the strength of the 5 balancing the particle size and-concentration of the reacting'materials, magnesia and phosphoric acid used. These will fall within the comparatively narrow limits set forth above .for general use and may be determined by experiment.

If the investment be mixed and used immediately, there will be a slightly lower expansion than if it be .mixed and worked until the initial set .has satin. By the use of aluminum the variation between the expansions under both conditions are greatly reduced andthe material may be used'iirmnediately or workedfor varying times l5 and still :pmduce accurate results.

When :the acid :acts on the magnesia, it may :form a plastic mass for a while and then set almost instantaneously. This would ometimes spoil the work since the model or upper half of the would be 'only partially poured when 'Egiijie' lie -.i' 'f hermeneut- To promote a st'l more even reactiop and a more regularizdsettih'gtiine,sinail amounts of magnesium phosphate are desirable and should be used pre era ly in amounts of 1% to 3% of the mixture. Less than 1% is generally preferable. This may be dissolved in the liquid or and magnesia gives a binder that is very reiractory. 'Other metal that form insoluble phosphates maybe added either to the liquid'or powder, these include principally zinc and cadmium but their use in appreciable quantities is undesirable since they seem to reduce the refractori- 40 uses of the mix. High amountsof algr inum act similarly.

The addition of small amounts of singlenor double zirconates and titanates when ground to be substantially all through}; mesh are sometimes of benefit in promoting greater plasticity to the mix and may be used up to approximately 3% of the total composition and generally should be less than half the amount of magnesia used. When magnesii z'rconi is used in amounts of 2% or 3%, the use of the small amounts of aluminum in the liquid preferably i more highTyhesirabldto prevent variable setting expansion. The use of these ziroonates and titanates also reduces the refractoriness somewhat and causes greater shrinkage of the mold often at high temperatures.

The magnesia may be used in its various forms, that is, in the preci itated form i nite The lat er is most desirablesince it has'alreadybeen shrunk to its .denses't state. The magnesia should be used in an amount .'-in excess of that required to react with .all 01' the prosphoric acid. Therefore, there will be an egpesggiunreacted magngsia present and when not in its densest form will promote shrinkage of the mold. The magnesia should be 200 mesh or finer. The finer the grind, the quicker the reaction and set. For most desirable working qualities, strength, etc.,

the magnesia should be in quantities more than acid Heat promotes nic'ierapid reaction; while cooling retards the setting.

' Instead of using a iiareaisstiasiemeilshr magnesia and 'a liquid consisting of the acid "and small amounts of alumina dissolved therein, a double mix wd may beused. The latter would consist of one wder conta' reiracto other powder wear as" me; esium' en. e proportioned and the mixture made plastic with water. Likewise, it is possible to add aluminum salts other than the hydroxide or phosphate in orderto have the aluminum present to reduce tlgg sgjgfiigigjgpghslon. It is preferable. however, to use an aluminum compound which 'williiorm the phosphate when added to the phosphoric acid and without liberating other compounds; such-as the chloride, -for example, would do.

The proportion of m agnesia used and the strength of the phosphoric acid solution control the setting expmn'fiaT'aansiHereeIe extent but when the higher percentages of both are used. the expansion begins to get out of control as explained above and here the aluminum compound controls it as desired. Whn'thmagnes'ia isifitfiemiddle of the limits used, the phosphoric acid should preferably be between 25% and 30% strength. The addition of some aluminum compounds, such as the hydroxide, into the phosphoric acid solutions decrease the free phosphoric acid content which also *tends to decrease the set- 'ting expansion somewhat. With the same strength of free phosphoric acid, the addition of aluminum still gives the desirable advantages of permitting the control of the setting expansion without'aiiecting the percentages of other ingre- I dients which otherwise give a good investment composition.

The use oforg-anic acids and temporary bind rs, such as gelatin. is"not recommended but may be 0 used where certain'efiects, such as slower settingcur-53y, the molds may time,is-desirecl.

The compositions were described above from the standpoint'of heating them to red heat and casting at elevated temperatures. Since the set; in sire-newsman treliedv ie as: WM, sed to reproduce objects accurately at room temperature and even castings could be made therein.

Due also-to the neutral setting expansion, it is possible to use the compositions described herein .for repairing refractories. The low setting expansion, combined with the other properties of these compositions, permits their adhering to other materialsand, since there is no expansion, there is no chance for distortion or cracking. Likewise.- these compositions could be used to makeaobjects, such as crucibles, in which metals could be melted.-

The above are typical examples of the uses of an investment composition utilizing the magn =sia-phosphoric aci d:alum inu m binder given herein and WWcoifr'seTpo'sTs'iblE"where greater strength is necessary to increase the binder in excess of the amount above set forth so long as the proportions-of each ingredient is maintained.

Likewise, the compositions given herein could be Examiner used for other purposes than those above set forth in which the properties are desirable.

The above are examples of the manner in which the magnesia-phosphoric acid-aluminum binder may be added to investments and should, therefore, be so considered and not as a limiting factor in the invention.

What is claimed is:

An investment composition suitable for making molds into which high melting point metals and alloys may be cast and consisting of age;

ctory filler makin up approximately 85% to \9% o einvestment, an e ga bin erconsisting of 3% to 10% of magesig, and

towhich is added an aluminum compound capable of forming the phosphate when mixed with phosphoric acid and being present in such amount as to contain .02% to 1% of aluminum, this investment being made into a plastic mass with a solution of phosphoric acid of 10% to 40% strength.

2. An investment composition suitable for making molds into which high melting point metals and alloys may be cast and consisting of a refractory filler making up approximately 85% to 97% of the investment, and the balance being a binder consisting of 3% to 10% of magnesia, this mixture being gaged into a plastic mass with a solution of phosphoric acid of 10% to 40% strength, and containing dissolved therein .02% to 1% of aluminum, based on the weight of the solid ingredients used.

3. An investment composition suitable for making molds into which high melting point metals and alloys may be cast and consisting of a refractory filler making up approximately 85% to 97% of the investment, and the balance being a binder consisting of 3% to 10% of magnesia, this mixture being gaged into a plastic mass with a solution of phosphoric acid of 10% to 40% strength, and containing dissolved therein sufficientaluminum to keep the setting expansion of the investment between .0% and .1% but said aluminum being present not in excess of 1% of the weight of solid ingredients used.

4. An investment composition suitable for making molds into which high melting point metals and alloys may be cast and consisting of a refractory filler making up approximately 85% to 97% of the investment, and the balance bein a binder consisting of 3% to 10% of magnesia, and to which is added an aluminum compound with phosphoric acid and beingv present in such mm .02% to 1% of aluminum, also .1% to 3% of tribasi c magnesium phosphate, this investment being made into a plastic mass with a solution of phosphoric acid of 10% to 40% strength.

5. An investment composition suitable for making molds into which high melting point metals and alloys may be cast and consisting of a refractory filler making up approximately 85% to 97% of the investment, of which refractory filler at least 5% is kyanite, and the balance of the composition being a binder consisting of 3% to of magnesia, and to which is added an aluminum compound capable of forming the phosphate when mixed with phosphoric acid and being present in such amount as to contain .02% to 1% of aluminum, this investment being made into a plastic mass with a solution of phosphoric acid of 10% to 40% strength.

6. An investment composition suitable ,for

making molds into which high melting point metals and alloys may be cast and consisting of a refractory filler making up approximately 85% to 97% of the investment, of which refractory filler at least 5% is kyanite, and the balance of the composition being a binder consisting of 3% to 10% of magnesia, and to which is added an aluminum compound capable of forming the phosphate when mixed with phosphoric acid and 10 being present in such amount as to contain .02%

to 1% of aluminum, also .l% to 3% of tri-basic magnesium phosphate, this investment being made into a plastic mass with a solution of phosphoric acid of 10% to 40% strength.

7. An investment composition suitable for filler at least 8% is zircon, and the balance of the composition being a binder consisting of 3% to 10% of magnesia, and to which is added an aluminum compound capable of forming the phosphate when mixed with phosphoric acid and being present in such amount as to contain .02% to 1 of aluminum, this investment being made into a plastic mass with a solution of phosphoric acid of 10% to 40% strength.

8. An investment composition suitable for making molds into which high melting point metals and alloys may be cast and consisting of a refractory filler making up approximately 85% to 97% of the investment, of which refractory filler at least 8% is zircon, and the balance of the 3 composition being a binder consisting of 3% to 10% of magnesia, and to which is added an aluminum compound capable of forming the phosphate when mixed with phosphoric acid and being present in such amount as to contain .02%

to 1% of aluminum, also .1% to 3% of tri-basic magnesium phosphate, this investment being made into a plastic mass with a solution of phosphoric acid of 10% to strength.

9. An investment composition suitable for making molds into which high melting point metals and alloys may be cast and consisting of a refractory filler making up approximately 85% to 1% of aluminum, this investment being made into a plastic mass with a solution of phosphoric acid of 10% to 40% strength.

10. An investment composition suitable for making molds into which high melting point metals and alloys may be cast and consisting of a refractory filler making up approximately to 97% of the investment, of which refractory filler at least 50% is silica,'and the balance of the composition being a binder consisting of 3% to 10% of magnesia, and to which is added an aluminum compound capable of forming the phosphate when mixed with phosphoric acid and being present in such amout as to contain .02% to 1% of aluminum, also .1% to 3% of tri-basic magnesium phosphate, this investment being made into a plastic mass with a solution of phosphoric acid of 10% to 40% strength.

ROBERT NEIMAN. 

